Interest in using copper as interconnects in semiconductor devices continues to increase since it possesses a lower resistivity and a reduced susceptibility to electromigration failure as compare to the more traditional aluminum or aluminum alloy interconnects.
However, since copper has a tendency when used in interconnect metallurgy to diffuse into surrounding dielectric materials such as silicon dioxide, capping of the copper is essential. The capping inhibits this diffusion. One widely suggested method of capping includes employing a conductive barrier layer along the sidewalls and bottom surface of a copper interconnect. Typical of such barrier layers is tantalum or titanium. Capping of the upper surface of a copper interconnect usually employs silicon nitride.
However, silicon nitride does not exhibit strong adhesion to copper surfaces despite various adhesion treatments. Accordingly, the silicon nitride-to-copper interface is susceptible to delamination under conditions of mechanical loading.
For example, to assure package reliability, the controlled collapse chip connection (C4) structural integrity must be sound in order to survive the mechanical stresses that the product experiences. Recent studies of C4 pads on copper interconnections reveal relatively weak C4 structural integrity. Failures arose during rework and burn-in operations due to the inherently relatively weak adhesion of the overlying silicon nitride cap to the copper.